In the field of digital printing it is known to have a printing apparatus having at least one print module and a transport arrangement defining a direction of transport and being arranged for transporting sheets past said at least one printing module for printing thereon. Sheets to be printed are typically fed to the transport arrangement from a paper feed device, which typically feeds sheets one by one into the transport arrangement. During the feeding of the sheets, it may occur that two (or more) sheets are fed into the transport arrangement in a superimposed manner, which occurrence is designated herein as a double sheet event. Such a double sheet event, if undetected, typically leads to a paper jam during the transport of the superimposed sheets through the printing apparatus. If such a paper jam occurs, all the sheets currently in the transport arrangement have to be manually removed and the printing apparatus has to be manually reset to allow a restart thereof. Even though double sheet events only occur sporadically, they may lead to considerable downtime of a printing apparatus during an extended period of time.
In order to avoid a paper jam due to a double sheet event and the need to manually remove the sheets from the transport arrangement, different approaches have been taken in the past.
In one approach, which was used in the NexPress 2001 “Classic”, a double sheet waste tray was used between a paper feed device and a print module of the NexPress 2001. Upon detection of a double sheet event, the superimposed sheets were diverted towards the waste tray in order to avoid a paper jam. Subsequently, all sheets in the transport path of the printing apparatus were purged into the proofing tray of the printing apparatus. After the purging, the printing apparatus was automatically stopped and the operator was notified. The operator then had to manually clear the proofing tray and had to manually reset the printing apparatus for a restart thereof. Even though this system may reduce down time of the printing apparatus due to the fact that sheets in the transport path did not have to be removed manually, its still requires substantial input and time of an operator. Furthermore, inasmuch as all sheets in the paper path are purged to the proofing tray in the above system, a large number of waste paper is generated.